1. Field of the Invention
The present invention relates generally to a liquid crystal compound, and more particularly to a liquid crystal compound used for producing a liquid crystal device using ferroelectric liquid crystal, and a ferroelectric liquid crystal composition, and a ferroelectric liquid crystal device using the liquid crystal compound.
2. Description of the Related Art
The sharp rise in demand for personal computers, especially the portable computers, has fueled the ever-increasing demand for liquid crystal displays in recent years. Liquid crystal televisions for home use are also gaining popularity, and as such the market for liquid crystal displays is continually expanding. The liquid crystal displays are now widely available in larger screen sizes, particularly in the case of the liquid crystal televisions for home use. In view of these trends, researches are in progress to further improve quality and to impart new functions to the liquid crystal displays.
Currently, the dominant design for liquid crystal displays includes: a TFT electrode substrate having TFT electrodes and pixel electrodes; a color filter substrate having a color filter layer to achieve color display; and a liquid crystal material is usually encapsulated between the TFT electrode substrate and the color filter substrate. These liquid crystal displays operate in various driving modes (such as TN, STN, MVA, IPS, and OCB) classified by the orientation of the liquid crystal material, of which TN, STN, and MVA mode liquid crystal displays are more widely used.
However, in the case of the liquid crystal displays as described above, the response speed of the liquid crystal materials is considered to be generally slow as it can be anywhere from several milliseconds to several tens of milliseconds. The slow response speed of the conventional liquid crystal displays speed leads to inferior performance of displaying moving images as compared to the conventional CRT displays or the PDPs that are also gaining popularity.
To increase the response speed in a liquid crystal display device, ferroelectric liquid crystal is gaining attention for its ability to provide the high response speed on the order of μs, which is much faster than that of the liquid crystal materials described above. It is expected that the liquid crystal displays utilizing the high response speed of the ferroelectric liquid crystal materials will one day be displaying moving images in a level of quality equal to that of the CRT displays or the PDPs.
The ferroelectric liquid crystal of a bistable liquid crystal has been known. A bistable liquid crystal has two stable states to show bistability when no voltage is applied, but a bistable liquid crystal cannot attain the graduation display. However, the ferroelectric liquid crystal in which the liquid crystal layer is stable in a single state (i.e., “monostable”) when no voltage is applied is gaining attention, as the monostable liquid crystal can provide graduation display by continuously changing the director (i.e., the inclination of the molecule axis) of the liquid crystal with a change in applied voltage (NONAKA, T., L I, J., OGAWA, A., HORNUNG, B., SCHMIDT, W., WINGEN, R., and DUBAL, H., 1999, Liq. Cryst., 26, 1599., also see FIG. 4). Some known monostable liquid crystal materials are:
(1) a ferroelectric liquid crystal in which phase change is caused in temperature lowering process between cholestric phase (Ch) and chiral smectic C phase (SmC*) via no smectic A phase (SmA); and
(2) a ferroelectric liquid crystal in which phase change is caused in temperature lowering process as Ch-SmA-SmC* and shows SmC* phase via SmA phase (see FIG. 5).
Therefore, the monostable ferroelectric liquid crystals made possible to obtain liquid crystal displays with faster moving images display characterstics.
Not only the above-described ability to ably display moving images, various other display quality characteristics, such as contrast, luminance and chromaticity characteristics, are also required in the modern liquid crystal displays. Some of these display quality characteristics sought to be enhanced vary depending on the driving mode of the liquid crystal display, but for enhancing contrast liquid crystal displays are required to use ferroelectric liquid crystals.
The liquid crystal display contrast depends on the degree of change in the alignment state of a liquid crystal material caused by application of voltage. Therefore, the contrast of liquid crystal displays using the ferroelectric liquid crystals could be enhanced mainly by adjusting the composition of the liquid crystal materials. More specifically, the liquid crystal displays using ferroelectric liquid crystals usually use a liquid crystal composition containing plural kinds of liquid crystal compounds, and therefore enhancement of the contrast of the ferroelectric liquid crystal displays is achieved by replacing one or more kinds of the liquid crystal compounds contained in the liquid crystal composition with other different kinds of liquid crystal compounds or by changing the composition ratio of liquid crystal compounds constituting the liquid crystal composition.
However, the conventional liquid crystal compounds have problems in that it is difficult to achieve desired contrast by changing the composition ratio of liquid crystal compounds or by selecting the kinds of liquid crystal compounds contained in a liquid crystal composition. In order to solve these and other problems, there is a need for providing liquid crystal compounds capable of drastically enhancing the contrast of a liquid crystal display using ferroelectric liquid crystal.